The present invention relates in general to wireless (e.g., cellular) communication systems, and is particularly directed to a base station-installed signal processing mechanism, which processes signals received at a phased array antenna to determine the angle of arrival of signals from a wireless communication device, such as a cellular phone, and correlates the base station""s transmitted supervisory audio tone (SAT) signal with that returned from the cellular phone, in order to determine range and thereby geolocate the device, as in the case of receipt of an emergency services access (xe2x80x98dial 911xe2x80x99) type of call.
Whenever a wireline telephone user places an emergency access call, such as by dialing 911, automatic number identification information (ANI) signals embedded in the call are employed by a database search mechanism to automatically provide the location of the calling subscriber circuit to a Public Safety Answering Point (PSAP). However, when a 911 type of call is placed from a wireless communication device, such as a cellular telephone, phone location information cannot be automatically provided to the PSAP, since the calling cellular phone is a mobile rather than a fixed piece of equipment. Because of this shortcoming, the Federal Communications Commission has mandated that steps be taken by the communications industry to make cellular phones geolocation capable to within a prescribed accuracy (one hundred twenty-five meters). In addition to meeting this emergency services requirement, automatically providing geolocation information also facilitates a number of ancillary functions, such as location-based billing, locating a stolen or misplaced phone, fraud reduction, and tracking and location capability for dispatch, delivery and sales personnel.
Current proposals to satisfy the above objectives include relatively complex and costly trilateration schemes that are based upon differences in time or angles of arrival of a timing fix or reference signal transmitted by the cellular phone and received at three or more geographically spaced apart receivers. Other proposals include the use of a phased array antenna and a pair of receiver stations to determine angle of arrival and difference in time of arrival for triangulation purposes. Another proposal is to require that all currently employed cell phones be retrofitted with additional signal processing circuitry, such as that including a global positioning system (GPS) processor-based receiver, through which the cell phone determines its position and then transmits that information as part of the 911 call.
In accordance with the present invention, rather than have to employ an arrangement of geographically dispersed receiver stations, or modify the internal circuitry of each cellular phone, advantage is taken of the supervisory audio tone (SAT) that is transmitted by a base station and returned from a calling cellular phone in the course of the call establishment process that is conducted via the control channel. In particular, the present invention is directed to a supervisory audio tone (SAT) signal processing mechanism that is readily installed in a respective base station equipped with a phased array antenna.
This SAT signal processing mechanism is operative to correlate the returned SAT signals with those transmitted by the base station to determine the range of the cellular phone. In addition, signals from the cellular phone, such as but not limited to the returned SAT signals, are subjected to an angle of arrival processing mechanism for determining the direction of the cellular phone relative to the base station. Given this angle of arrival and range information the cellular phone is geolocated.
The transceiver of a base station is coupled with a supervisory audio tone (SAT) transmitter, which is operative to generate a supervisory audio tone, and an SAT receiver, which is operative to receive and perform standard processing of the SAT signal returned by a loopback mechanism embedded within the calling cellular subscriber""s mobile phone and received by the base station transceiver in the course of setting up the call prior to voice communication signaling.
Radio wave signaling is effected via a phased array antenna. The elements of the antenna are coupled through down-conversion and digitization circuitry to a digital signal processing unit, which subjects signals received by the respective elements of the array to an angle of arrival determination routine, to provide a directional (azimuth) fix on the location of the calling cellular phone relative to the base station.
In order to determine the range of the calling cellular phone relative the base station, each of the SAT signal sourced by the SAT transmitter and the returned SAT signal received by the base station radio is subjected to a correlation processing routine, coupled to the output of a band pass filter tuned to the frequency of the supervisory audio tone. The largest of correlation peak values is selected by a peak detector as that associated with the travel time and therefore line-of-sight distance or range to the cellular phone.
Since the total travel distance of SAT signals between the base station and the cellular phone traverses a two-way path from the base station to the cellular phone and back, plus a loopback path through the circuitry of the phone, the time offset associated with the correlation peak selected by the peak detector is slightly more than twice the actual travel time of radio wave signals between the cellular phone and the base station.
To correct for this offset through the circuitry of the mobile phone, the actual range of the cellular phone from the base station is calculated in accordance with the equation R=Cxm/2xe2x88x92k, where C is the velocity of light and k is a correction factor associated with the loopback path through the circuitry of the cellular phone, and varies with manufacturer and phone type. The value of k is provided by the phone manufacturer and is stored in a database; it is accessed using equipment serial number information conveyed in the incoming control channel call establishment request from the calling phone.